Many four-wheel drive vehicles are equipped with a transfer unit which distributes motive power from the powertrain to the front and rear axles in order to drive all of the wheels. For example, in front-wheel drive vehicles, a power take-off unit (PTU) is used to transfer power from the transaxle to the rear driveline. One known type of PTU includes a transfer shaft driven by the transaxle and an intermediate shaft which is driven by the transfer shaft. Typically, the intermediate shaft is provided with a bevel gear which meshingly engages a second bevel gear mounted on the PTU output shaft which drives the rear driveline. A, problem with current bevel-type transfer units is that the bevel gears are large and can generate undesirable noise.
Accordingly, it is desirable in the art of vehicle powertrains to provide an alternative to current transfer units which are smaller and quieter than current designs.
Worm gearsets, otherwise referred to as worm/worm gear transmissions, are known in the mechanical power transmission field. The worm gear is driven by the rotation of the worm with which it meshes. The rotational speed of the associated shaft of the worm gear is a function of the number of teeth on the worm gear and the number of threads on the worm. The worm may be single or multiple threaded. Conventional worm/worm gear transmissions have a worm gear with at least 24 teeth. In particular, the American National Standard “Design of Industrial Double-Enveloping Wormgears” (ANSI/AGMA-6030-C87) recommends 24 as the minimum number of gear teeth. Furthermore, the enveloping angle of known worm gears for one revolution of the thread of the worm is not more than 15 degrees. Moreover, standard double enveloping worm/worm gear transmissions have been used only for ratios of five and more. Due to such high ratios, it has been considered impractical to use the worm gear as the driven member and the worm as the driving member to transfer power from the worm gear to the worm.